The invention relates to self-locking threaded inserts which are threaded into a tapped hole to replace or strengthen worn or weakened threads.
Many different inserts have been developed together with various approaches for installing them. One such approach is the use of spiral metal coils threaded into the base material with the coil having a diamond-shaped cross-section. While such coils improve strength they are limited by the size of diamond-shaped wire which must match the pitch of the threaded hole that receives the coil. The approach of threading one wire coil into a prepared hole and then threading another coil into the first one is sometimes used to achieve smaller diameter. This is a tedious and expensive process.
Another system includes the use of a threaded insert having axial slots in its exterior and pins initially attached to the insert near its upper end, to be pounded into the base material after the insert has been threaded into the tapped hole in the base material. As can be appreciated, this system is expensive from both a fabrication and installation standpoint. In addition, the pins sometimes break off before installation, thus rendering the insert unusable, or the pins may deform the thread in the tapped hole.
Inserts having exterior threads which interfere with the threads in the tapped opening of the base material have also been employed, but this solution has been unsatisfactory due to difficulties of installation and reliability of staying in place.
Another approach is described in U.S. Pat. No. 3,566,947, issued to the instant inventor which is incorporated by reference herein. This patent discloses a thin walled threaded metal insert which has a threaded exterior and a partially threaded interior. A threaded tool is then used to position the insert in the tapped hole and complete the thread on the interior portion of the insert. This causes a bulging of the insert walls outwardly to lock the insert in the tapped hole. Although such an insert is quite suitable for many applications requiring thin walled inserts, in other applications, a thick walled insert is required.
Applications requiring a thick walled insert are of two basic types. The first is an insert repair situation in which the threads of an installed insert have been stripped. Such a problem can exist, for example, with inserts used to provide threaded connections for spark plugs. In order to repair the connections, the old thread must be drilled out and the hole counterbored and tapped. This creates a tapped hole of a larger diameter than originally present, which in turn requires an insert of a larger diameter. Since the threaded connection on the spark plug remains the same, the wall of the new insert must be thicker in order that the interior diameter of the new insert be the same as that of the old insert.
The second major type of application for thick walled inserts is an original threaded connection situation in which the depth of the tapped hole is small such as occurs when the base member is thin. Since the depth of the tapped hole is small, in order to create a threaded connection with sufficient strength, the surface area of contact between the tapped hole and the threaded member which is to be inserted in the tapped hole must be increased. This can be accomplished by using a large diameter insert which will increase the pullout strength of the theaded connection. Since the diameter of the screw or bolt which is to be threaded into the tapped hole is of a predetermined size, often the insert must have a thick wall.
These applications requiring thick walled inserts present a special problem to the inserts described in U.S. Pat. No. 3,566,947. This is true because the walls of the inserts are of such a thickness that when the interior thread is completed by a threaded tool, the force created is not sufficient to bulge the insert walls outwardly far enough to lock the insert securely in the tapped hole. The disclosed invention is particularly adapted to solve these problems associated with thick walled inserts.